Recent research has shown that high accuracy molecular orbital methods can give considerable insight into the energetics of intermolecular interactions and structure of intermolecular complexes. We propose to apply these theoretical methods to problems in ion hydration, hydrogen bonding, steroid biological activity and solvation of organic intermediates. Some specific applications will be the determination of the structure, energetics, charge distribution changes and IR properties of hydrated anions and cations, an extension of our knowledge about the nature of the hydrogen bond and the H-bond potential function in peptides and proteins, a rationalization of some steroid biological activities (particularly the role of F-substitution) and the development of a suitable solvation model for organic intermediates. Some of these results might have wider biological applications in the areas of membrane transport, coenzymes and structure-activity relationships.